ui.backend.gtk: better gesture for shift tab

gdk emits ISO_Left_Tab for shift + tab. Not sure if factor should silently
transform that in tab..
With this patch, shift tabs emits the following gesture:
T{ key-down { mods { S+ } } { sym "TAB" } }

basis/ui/backend/gtk/gtk.factor

@@ -113,6 +113,7 @@ CONSTANT: action-key-codes
     H{
         { $ GDK_KEY_BackSpace "BACKSPACE" }
         { $ GDK_KEY_Tab "TAB" }
+        { $ GDK_KEY_ISO_Left_Tab "TAB" }
         { $ GDK_KEY_Return "RET" }
         { $ GDK_KEY_KP_Enter "ENTER" }
         { $ GDK_KEY_Escape "ESC" }